With the advent of digital television, a need has arisen for broadband multichannel antennas suitable for radiating signals at UHF wavelengths. In some jurisdictions, various regulations require stations to begin broadcasting digital signals and also to continue broadcasting analog signals. Stations may be assigned frequency bands that are close together or far apart for such multiple broadcasts. As a result, it would be extremely useful if a broadband antenna could be developed that would radiate efficiently from 470 MHz to 860 MHz, the UHF television band, and would also have a low VSWR at its input terminal in that band. Typically, a VSWR of about 1.15 or less is required in a television transmission system.
Dipole arrays may be designed that are capable of broadband operation across the UHF band and that meet the VSWR requirements for television transmission systems. However, there are disadvantages to using dipole arrays. A dipole array assembled in a large panel on a tower has a large wind load and tends to be less robust mechanically than other antenna designs. Dipole arrays also tend to be mounted on large structures to accommodate the mechanical loads of the panels. This makes it hard to achieve desirable radiation patterns for the antenna system.
Slot antennas have been used in physically demanding environments such as on airframes for narrow bandwidth applications with success. Slot antennas, however, have not been developed that can operate across the UHF band with the low input VSWR required for a television transmission system. A waveguide slot antenna consists of a length of waveguide short circuited at one end and open circuited at the other. The open end is usually terminated in some type of ground screen, and the antenna is excited by a coaxial to waveguide transition.
FIG. 1 is a diagram illustrating a slot antenna with a crossbar transition between the coaxial cable to the waveguide. A slot antenna 100 is shown. A waveguide 102 is closed on one end and open at open end 103. Slot antenna 100 is fed by a 50-ohm coaxial cable line 104 that is connected to the top of waveguide 102. The outside of the coaxial line is electrically connected to the waveguide. The center conductor of the line is electrically connected a T-shaped bar 106 that extends downward into the waveguide cavity. T-shaped bar 106 includes a cross member 108 that extends the length of the cavity and is terminated at the sides 110 of the cavity. Such an antenna is generally referred to as a T-bar fed slot antenna.
Such antennas have been the object of considerable study in the prior art. A cavity backed rectangular slot antenna is described in Reference 1, "Antenna Engineering Handbook", Henry Jasik, First Edition, McGraw-Hill, 1961, which is herein incorporated by reference for all purposes. The original design work was published in Very High Frequency Techniques, compiled by the Radio Research Laboratory and published by McGraw-Hill in 1947. The VSWR of two T-bar fed slot antennas investigated is shown in FIG. 2A. The VSWR results for the earlier investigation of such T-bar slot antennas is shown in FIG. 2B.
Another investigation of the design of T-bar fed slot antennas is provided in Reference 2, "Some Important Parameters in the Design of T-Bar Fed Slot Antennas, by E. H. Newman and Garry A. Thiele, IEEE Transactions on Antennas and Propagation, January 1975, pages 97-100, which is herein incorporated by reference for all purposes. The VSWR of a tuned T-bar fed slot antenna is shown in FIG. 2C. Newman, et al., further describes how to design T-bar fed slot antennas for various bandwidths.
Although the performance of such antennas described in the prior art is good, the VSWR across a broad bandwidth of 1.8 .lambda. corresponding to the UHF frequency spectrum is too large. The VSWR is greater than 1.5 at several points and in particular, tends to increase substantially at high frequencies within its operating band. The VSWR of the antennas described in FIGS. 2A and 2B increases to above 2 at the high frequency end and even the tuned antenna shown in FIG. 2C has a peak VSWR of greater than 2 at the high frequency end of its operating band.
In spite of the considerable work that has been done on T-bar fed slot antennas, the performance of such antennas has not been improved to the point where such antennas may be used with success across the entire UHF frequency spectrum for television transmission. Other techniques are needed to enable the use of slot antennas for such applications. It would be useful if a slot antenna could be developed that could meet the design requirements for television transmission across the UHF band.